This invention relates to systems for supplying breathable oxygen, and more specifically to relief valves for systems for supplying oxygen for breathing in an aircraft cabin.
The term xe2x80x9cpoppetxe2x80x9d is used here to refer to a single pressure-actuated valve mechanism
An oxygen storage pressure relief system monitors a bank of oxygen cylinders, perhaps as many as twenty, which supply oxygen to the passenger and crew compartments of a medium size or large aircraft. The purpose of the relief system is to prevent an overpressure condition in both the lines from the oxygen cylinders and the manifold line, by opening the line under conditions of excess pressure and venting oxygen outside the aircraft cabin just until the overpressure condition is relieved. A buildup of pressure in the lines could break a line and flood the fuselage with pure oxygen causing a fire risk.
In conventional oxygen storage cylinder systems, such as that described in U.S. Pat. No. 5,159,839 (Silber et al.) there is a relief valve, made up of a single poppet valve, on each pressure reducer. See FIG. 1. Such prior art systems put a system relief valve 10 between the pressure regulator of each O2 cylinder 15 and the relief manifold line 19, so that the single relief manifold line 19 carries oxygen for all cylinders 15. See FIG. 1a for detail. Each cylinder 15 has a DOT-required pressure relief burst disc 11 which is upstream of relief valve 10. Relief valve 10 is located at the outlet of a pressure regulator or reducer 12 which is mounted directly to the cylinder hand valve 14. If there are twenty cylinders 15, there are twenty relief valves 10.
See FIG. 1b, which shows a single operating case of the system of FIG. 1. This prior-art approach does not provide relief of overpressure when a valve 10 on a single cylinder 15 is stuck in a shut position, thereby preventing relief of overpressure in that cylinder. The stuck-shut case is a single-point failure case, in that the behavior of the system as a whole is degraded if only one failure occurs. The probability p of failure of a single relief valve may be very small, but in a prior-art system such as that in FIG. 1 with twenty oxygen cylinders all in active service, the probability of failure of any single valve is just under twenty times p. This approximate relationship is expressed in exact form as: nxc3x97(1xe2x88x92p)nxe2x88x921xc3x97p, where n is the number of valves and p is the probability of failure of a valve. For low individual-cylinder failure probabilities, the relationship holds in nearly linear fashion as the number of actively-serving oxygen cylinders increases.
A second failure mode of a relief valve occurs when it leaks or remains wide open, allowing the individual cylinders to bleed to zero psig. See FIG. 1c. When a valve 10 on a single cylinder 15 is stuck in an open position, it vents oxygen freely. Like the stuck-shut case, the stuck-open case is a single-point failure case, in that the behavior of the system as a whole is degraded if only one failure occurs.
This near-linear increase in the probability of a single-point failure makes larger prior-art systems more vulnerable to frequent valve failure and its system-wide consequences. A better-designed system would display reduced frequency of valve failure, and would restrict the consequences to the system whenever any such failure occurs.
Other prior-art systems, such as that described in U.S. Pat. No. 4,148,311 (London et al.) do not even address the problem of oxygen overpressure in a system with multiple oxygen cylinders as used in large aircraft. There is a clear need for an expandable, reliable, inexpensive oxygen pressure relief system for aircraft use.
The invention is a reliable and economical apparatus for relieving pressure in a large aircraft cabin oxygen supply, where multiple oxygen cylinders are used concurrently. The invention uses a series-parallel array of valves actuated by changes in differential pressure between the oxygen supply and the ambient cabin atmosphere. The series connection of its valves reduces the risk of open-valve failures, while the parallel connection of sets of series-connected valves reduces the risk of closed-valve failures. The small number of valves used in its design reduces the cost of the invention. The invention""s series-parallel structure is optionally extended to larger numbers of valves to facilitate the use of less-expensive valves and supporting components without loss of reliability.